WO99/41155 discloses a device for producing container blanks by interconnecting opposite wall portions of a material web along connecting portions.
The material web comprises two side wall webs which are passed in a parallel opposite relationship along a manufacturing line and, arranged therebetween, a bottom wall web folded in two. The webs are thus joined to form a common web of material and are sealed to each other along said connecting portions by sealing tools which engage the web of material.
It is also possible to let the material web consist of a web which is folded longitudinally in the form of an M. A container blank produced from such a material web will obtain two side walls from the outer lateral portions of the web and a bottom wall from the central portion, folded in two, of the web.
A problem that may arise in said sealing along the connecting portions relates to their reliability. In fact, it has been found that it is difficult to provide a reliable seal in the transition between the areas of the material web which comprise two layers of material and the areas which comprise four layers of material. If the pressure is not sufficient, a duct extended parallel to the transition tends to be formed, which obviously results in the container blank not being liquidproof.
On the other hand, if the pressure is dimensioned so as to prevent the forming of said duct, the pressure must be so high that the bottom wall forming part of the material web is subjected to strain in the form of stretching and/or buckling which may result in breakage.
There is thus a need for a sealing method which eliminates the above-mentioned problems.